By the middle of the 20th century scientists believed they were well on the way to treating,preventing,or eradicating many of the deadly infectious diseases that had plagued humankind for centuries.
应该: you have to do something about it. it, one/ones, that/those 都 作代词 it 往往指文现同事物复数用they/them;one/ones 代替与前面提事物属于类 并同（些）数名词表示泛指意义；that代替同类非同数名词单数或数名词 表示特指意义 能指物能指 其复数those, 指物指简言：it 同 one 同类 that 同类 表特指